In order to produce fluids such as oil, gas, and water from subterranean rock formations, a well is drilled into the fluid-bearing zone. Most wells are generally drilled with a drilling rig, a drill bit, a drill pipe, and a pump for circulating fluid into and out of the hole that is being drilled. The drilling rig rotates and lowers the drill pipe and drill bit to penetrate the rock. Drilling fluid, sometimes referred to as drilling mud, is pumped down the drill pipe through the drill bit to cool and lubricate the action of the drill bit as it disaggregates the rock. In addition, the drilling fluid removes particles of rock, known as cuttings, generated by the rotational action of the drill bit. The cuttings become entrained in the column of drilling fluid as it returns to the surface for separation and reuse. The column of drilling fluid also serves a second purpose by providing weight to prevent seepage from the formation into the well. When the weight of the column of drilling fluid is used to prevent seepage, the hydrostatic pressure of the column of drilling fluid exceeds the pressure contained within the formation, a drilling condition referred to as over balanced drilling.
A desired condition when drilling is to prevent drilling fluids from penetrating the surrounding rock and contaminating the reservoir. Another desired condition is to allow any fluid such as oil from the reservoir being drilled to flow into the well bore above the drill bit so that production can be obtained during the drilling process. Both of these conditions can be achieved by lowering the bottom hole pressure, or in other words, lowering the hydrostatic pressure that is exerted by the column of fluids in a well bore to a point that is below the pore pressure which exists within a rock formation. Lowering the bottom hole pressure within a well bore while drilling below the formation pressure to accomplish either of these goals is called under balanced drilling.
Conventional under balanced drilling intentionally reduces the density of fluids contained in the well bore. In conventional under balanced drilling, the reduction in the density of the fluids causes the hydrostatic pressure of the fluid column to be lower than the pressure contained within the pores of the rock formation being drilled. When a reduction in density causes the hydrostatic pressure of the fluid column to be lower than the pressures contained within the pores of the rock formation being drilled, fluids in the reservoir may flow into the well bore while it is being drilled. Under balanced drilling has gained popularity in the upstream oil and gas industry because it does not allow the drilling fluids to penetrate the surrounding rock and damage the permeability of the reservoir.
The under balanced condition is usually achieved by injecting a density reducing agent such as air, nitrogen, exhaust, or natural gas into the fluids that are being pumped down the drill pipe during the process of drilling a well. The injected gas combines with the drilling fluid and reduces its density and thus lowers the hydrostatic pressure that exists in the annulus between the drill pipe and the wall of the well bore. The concentric casing technique is a common method for delivering the gas to the bottom of the well by utilizing a second string of casing hung in the well bore inside the production casing. The injected gas flows down to the bottom of the well through the outer annulus created by the two strings of casings. The drilling fluid, delivered via the drill pipe, and any produced fluid combine with the injected gas as it flows upwards through the inner annulus between the second or concentric string of casing and the drill pipe. The process may be reversed such that the inner annulus is used for injection and the outer annulus is used for well effluent. The use of gas as a density reducing agent has distinct disadvantages. First, if air is used, the risk of down hole fires and corrosion problems are invited. Second, if an inert gas such as nitrogen is used, the expense may be prohibitive. In either case, the cost of compression that is required by all types of gas at the surface is significant.
Another method for lowering bottom hole pressure is by artificially inducing lift to remove fluids from a well by using a jet pump and a power fluid. The use of jet pumps is common in production operations where drilling activity has stopped. In this case, the drill pipe and drill bit have been extracted and a jet pump is lowered into the well on the end of a tubing string. A surface pump delivers high-pressure power fluid down the tubing and through the nozzle, throat, and diffuser of the jet pump. The pressure of the power fluid is converted into kinetic energy by the nozzle, which produces a very high velocity jet of fluid. The drilling and production fluids are drawn into the throat of the jet pump by the stream of high velocity power fluid flowing from the nozzle into the throat of the jet pump. The drilling and production fluids mix with the power fluid as they pass through the diffuser. As the fluids mix, the diffuser converts the high velocity mixed fluid back into a pressurized fluid. The pressured fluids have sufficient energy to flow to the surface through the annulus between the production casing and the tubing that carried the jet pump into the well.
While jet pumps are used for removing fluid from a well by lowering down hole pressure in production wells, the advantages of under-balanced drilling would be enhanced significantly if a jet pump could be combined with drilling operations. The jet pump could be employed to achieve under-balanced conditions while the drill string is down in the hole and the drill bit is operating. By using a power fluid such as water, the disadvantages of gas could be avoided altogether thereby increasing safety and decreasing costs. Attempts have been made to place jet pumps into drill bits. However, when the jet pump is placed in the drill bit, the drilling fluid serves a dual purpose and becomes the power fluid before entering the nozzle of the jet pump. When the power fluid and the drilling fluid are one in the same and enter the nozzle of the jet pump, the extreme abrasiveness of the drilling fluid can cause the jet pump to wear out prematurely.
What is needed beyond the prior art is a jet pump connected to a concentric casing string that will induce artificial lift while allowing the drill bit to operate independently of the jet pump. What is further needed beyond the prior art is a jet pump connected to a concentric casing string that will keep the power fluid separate from the drilling fluid until after the power fluid has passed through the nozzle of the jet pump.